Characterization of Effluent Fractions from CI02 and CI2 Bleaching of Unbleached and O2 Bleached Softwood Kraft Pulps, 1993 Pulping Conference Proceedings

Effluents from OC(EO), OD(EO), and D(E0) laboratory bleaching of softwood kraft pulp were characterized by fractionation and analysis of the fractions. Adsorbable organic halide (AOX) and total organic carbon (TOC) were determined for the effluents and effluent fractions from each stage, with the exception of the oxygen stage. Each fraction was characterized in terms of its size, and in terms of its ratio of chlorine to carbon - an environmentally significant parameter. The fractionation consisted of ether extraction followed by separation of the extract into acidic, phenolic, and neutral subfractions.

Replacement of chlorine with chlorine dioxide after oxygen bleaching sharply reduced the AOX, TOC, and chlorine to carbon ratio (expressed as the number of chlorine atoms per 100 carbon atoms, Cl/C100) of the whole effluents. Larger AOX and TOC reductions were seen in the ether soluble fraction and in the phenolic subfraction of the ether soluble material. Since both Cl/Cl00 and the size of the ether soluble fractions may be expected to correlate with the potential of an effluent for negative environmental effects, these observations show that replacement of chlorine by chlorine dioxide may be more beneficial than the resulting reductions in whole effluent AOX would suggest.

Oxygen delignification prior to a 100% chlorine dioxide stage reduced whole effluent AOX and TOC in rough proportion to the amount of lignin removed in the oxygen stage, but did not affect the overall Cl/C100. The reductions in AOX and TOC in the environmentally significant ether soluble neutral and phenolic fractions were slightly larger. There was also a significant reduction in Cl/Cl00 in these fractions. Oxygen delignification, like chlorine dioxide substitution, may therefore be more beneficial than the overall AOX reduction would suggest.